1. Field of the Invention
The present invention relates to optical firs, and more particularly to long-period optical fiber gratings and methods of manufacture thereof.
2. Description of the Related Art
Long-period fiber gratings (LPFGs) have recently been attracting much attention for their possible applications such as gain-flattening filters for erbium-doped fiber amplifiers. Most LPFGs have been written in germanosilicate optical fibers because periodic structures can be easily obtained using UV-induced refractive-index changes due to the photosensitivity of Ge-related glass defects. This LPFG writing technique requires that fibers for LPFGs contain photosensitive sites, and thus the technique cannot be applied to those fibers which have no photo-reactive centers inside, such as pure-silica-core optical fibers.
In a general if fluorine is doped into a silica-core of a fiber, the refractive index of the core is reduced. Also, it is a matter of course that a stress is present during the preform stage, which is due to the difference between thermal expansion coefficients of the core and the cladding. If an optical fiber is formed by drawing the preform, stress is generated at the core and the refractive index thereof is reduced. In other words, a higher drawing tension leads to an increase in the tensile stress in the high viscosity core region, which is due to the photoelectric effect. By annealing the optical fiber, the residual stress can be easily released and the refractive index is restored to the level of that at the preform stage.
In fabricating a long-period optical fiber grating by annealing the optical fiber having the above-decribed configuration, in order to control the characteristics of the long-period optical fiber grating, the amount of fluorine (F contained in the optical fiber, or the drawing tension of the optical fiber, is adjusted. However, it is not easy to obtain an optimum long-period optical fiber grating just by adjusting the amount of fluorine (F) contained in the optical fiber, or the drawing tension of the optical fiber.
In order to guide light through a core, the refractive index of the core must be higher than that of the cladding around the core. In another method for writing long-period optical fiber gratings in a conventional optical fiber, the long-period optical fiber grating is written such that an optical fiber comprised of a N.sub.2 -doped core and a cladding made of SiO.sub.2, is annealed by arc discharge or CO.sub.2 laser irradiation. However, according to this manufacturing method, since N.sub.2 escapes from the core over a period of time, the reliability of the fiber grating may be degraded.
In a method for writing long-period optical fiber gratings in another conventional optical fiber, as discussed above, the refractive index of the core must be higher than that of the cladding around the core. The long-period optical fiber grating is written such that H.sub.2 is injected into an optical fiber comprised of a SiO.sub.2 --GeO.sub.2 core and a cladding made of SiO.sub.2, and then a UV laser irradiates the fiber. However, according to this manufacturing method, the life time of the long-period optical fiber grating is not long and thus the reliability is not ensured.
Additional examples of optical fibers and long-period optical fiber gratings of the conventional art are seen in the following U.S. Patents. U.S. Pat. No. 4,435,040, to Cohen et al., entitled DOUBLE-CLAD OPTICAL FIBERGUIDE, describes a W-proflle optical fiber with a core an inner cladding and an outer cladding, with the claddings being fluorine-doped. U.S. Pat. 4,822,399, to Kanamori et al., entitled GLASS PREFORM FOR DISPERSION SHIFTED SINGLE MODE OPTICAL FIBER AND METHOD FOR THE PRODUCTION OF THE SAME, describes a glass fiber preform with an inner core made of GeO.sub.2 --SiO.sub.2 or GeO.sub.2 --F--SiO.sub.2, an outer core made of F--SO.sub.2, and a cladding made of F--SO.sub.2. U.S. Pat. No. 5,568,583, to Akasaka et al., entitled DISPERSION COMPENSATING OPTICAL FIBER FOR WAVELENGTH MULTIPLEX TRANSMISSION AND METHOD USING SAME, describes an optical fiber with a W-shaped refractive index distribution, with a germanium-doped core, a fluorine-doped internal clad layer and a silica outermost clad layer. U.S. Pat. No. 5,673,354, to Akasaka et al., entitled DISPERSION COMPENSATING OPTICAL FIBER, also describes an optical fiber with a W-shaped refractive index distribution. The multi-cladded fibers of the above mentioned patents are designed for dispersion compensation, however, and their use in a long-period grating is not described. U.S. Pat. No. 5,892,615, to Grubb et al, entitled OUTPUT POWER ENHANCEMENT OPTICAL FIBER LASERS, describes an optical fiber with multiple claddings and further constructed with a core with a long period grating formed therein The long period grating is formed in the core using conventional techniques, and the above problems associated with conventional techniques obtain.
Based on our reading of the art, then, we have found that what is needed is a long-period optical fiber grating which is stable over a long time. Also, a method is needed for making a long-period optical fiber grating not relying on photoreactive sites in the core of the fiber.